Refinery offgases, typically offgases from fluid catalytic cracker units and coker units, contain quantities of olefins which can be economically recovered. Many times this recovery is integrated with existing olefins plants but in certain instances where offgas flow rates are large enough, stand-alone units have also been operated. Because of the higher quantity of lighter components such as hydrogen, nitrogen and methane, the feed gases are typically compressed from pressure of about 1.17 to 1.38 MPa gauge (170 to 200 psig) to pressures around 3.45 MPa gauge (500 psig) in multi-stage feed gas compressors. The compression step allows for the recovery of 90% to 99% of the ethylene and heavier materials contained in the feed gases using a combination of mechanical refrigeration and expansion of the methane and lighter portions of the feed gas after demethanization. However, the capital and operating costs for the feed gas compressors are very high.
The processing of refinery offgases for olefin recovery has associated safety concerns since nitrogen oxide is also present in trace amounts in the refinery offgas stream. The nitrogen oxide easily oxidizes forming nitrogen dioxide which can form solid nitrogen peroxide (N.sub.2 O.sub.4) at temperatures below -102.degree. C. N.sub.2 O.sub.4 and heavier diolefins (C.sub.4 +) can react at these low temperatures forming nitrated gums which are unstable and can explode if thermally or mechanically shocked.